In layer-forming phosphating of metals for corrosion protection, rinsing is often performed after phosphating, using aqueous solutions that contain complex zirconium fluorides. This so-called “post-treatment rinse” or “post-passivation” improves the corrosion protection produced by the phosphate layer. After this post-passivation step, the metal surfaces treated in this fashion are rinsed with water. This produces zirconium-containing rinse water that was hitherto sent on for wastewater treatment.
Rinse water from phosphating processes, which contains heavy metals and, in the context of corresponding post-passivation, zirconium as well, is usually treated by adding a suspension of calcium hydroxide (“lime water”). The concentration of partly dissolved, but predominantly undissolved suspended calcium hydroxide in these suspensions is usually in the region of 10 wt %. The pH of the rinse water rises in this context into a range from approximately 9 to approximately 11. Heavy-metal and zirconium ions dissolved in the rinse water thereby precipitate as hydroxides and phosphates, and are separated out, together with excess calcium hydroxide, as a slurry. Because of the excess of dissolved calcium ions and of OH− ions, the conductivity of the wastewater pretreated in this fashion is usually approximately 1000 to approximately 10,000 μS/cm. After this pretreatment, and after neutralization if necessary, the rinse water is sent on to the wastewater treatment plant. Rinse water is thus discharged from the water circulation system of the phosphating facility, and generates disposal costs. The water circulation must be correspondingly topped up with fresh water, which also entails costs. Because of the high ion load (reflected in the high conductivity) of the rinse water after lime water precipitation, it is not economical to use, for example, ion exchange or reverse osmosis to produce softened or demineralized water that can be reused in the water circulation system of the phosphating facility. The object of the present invention is to make available a method with which zirconium-containing rinse water can be processed in such a way that it can be reused in the phosphating process as rinse water.